Polymer-containing latexes, such as e.g. SBR (styrene-butadiene rubber) or NBR (acrylonitrile-butadiene rubber), polychloroprene, polybutadiene, polyisoprene, natural rubber latex, polyvinyl chloride, (meth)acrylate dispersions or dispersions of copolymers thereof, are conventionally available commercially in solids concentrations of over 45 to 50%. These concentrations can be achieved e.g. by increasing the concentration of low-concentration polymer latexes. The concentration of natural rubber latex is increased from 30% to 60% solids, for example.
Various methods of increasing the solids content are known in the prior art. In Houben-Weyl, Volume XIV/1, Makromol. Stoffe Part I, 4th edition, p. 515 (1961) or Polymer Colloids, Elsevier Applied Science Publishers, p. 272 (1985), for example, various possibilities are described, such as the addition of electrolytes, freezing out, evaporation of water, ultrafiltration, centrifugation or the addition of creaming agents.
These methods for increasing the solids content of polymer latexes are in some cases associated with considerable disadvantages, however. Thus, in the case of filtration, for example, there is always a risk that the pores of the filters will become blocked, resulting in an irreversible coagulate.
Centrifugation, which is used on an industrial scale for natural rubber latex in particular, is also problematic for synthetic latex as the particle sizes are often very much smaller than they are in natural latex. Another disadvantage of the centrifugation process is that the centrifuge can often be sealed only inadequately. This can cause air to be introduced into the aqueous phase, leading to foam formation, as a result of which throughput cannot be achieved in the desired order of magnitude.
Synthetic latex can be concentrated by creaming, for example. This is generally achieved by the addition of an aqueous solution of a creaming agent, for example sodium alginate, cellulose derivatives, methyl cellulose, agar-agar, gelatine, glue, pectin, salts of polyacrylic acid or by additional introduction of calcium acetate solution. It is also said that additions of ethylene oxide-containing non-ionic emulsifiers favour an increase in solids content.
Another important variable which is critical for the success of creaming and the creaming rate is the size of the latex particles. The larger the latex particles, the better the creaming process and the higher the latex solids content that can be achieved. In some circumstances it can therefore, be necessary to increase the size of the latex particles by pretreating them with e.g. NaCl solution.
The influence of particle size on the increase of the solids content is described for example in Ind. Eng. Chem., 43, 407 (1951). It emerged that a styrene-butadiene latex with a particle size of 78 nm could not be concentrated at all by addition of 0.1% ammonium alginate (relative to the aqueous phase), whereas a latex with a particle size of 360 nm could be concentrated very effectively.
Another important influencing variable is the ratio of creaming agent to latex quantity. If the ratio of creaming agent to latex quantity is too great, the latex is only thickened and the concentration process is slowed down. Too small a quantity of creaming agent on the other hand brings about no increase in solids content at all.
It is also said, for example, that the increase in solids content can be achieved by the addition of acids. In the case of latexes that have been stabilized by means of anionic emulsifiers, however, a shift to lower pH values increases the risk of the latex coagulating irreversibly. Such concentration processes are described for natural rubber latex, polystyrene-butadiene latex, polychloroprene latexes, copolymers of polyvinyl chloride and of polyvinylidene chloride.
In the case of polychloroprene latexes an increase in concentration from approx. 30% solids to 55% to 60% solids content has so far been achieved in industry by concentration with a creaming agent such as e.g. Na alginate. However, the rate of creaming varies enormously with the particle size. The smaller the particle size, the slower the concentration process, such that creaming times of up to 14 days arise with particle sizes below 100 nm.